Some of the advantages of on-line automatic measurement of ammonia, nitrate and phosphate for studying activated sludge systems are pointed out with the help of examples of batch experiments. Sample taking is performed by cross-flow filtration and measurement of all three analytes is performed by Flow Injection Analysis (FIA). Two batch set-ups are described. In the first, one of the two 800 1 nitrifying/denitrifying tanks of a pilot-scale alternating process is employed as batch reactor, which has the advantage of a high measurement frequency and little preparatory and clean-up effort. The second consists of four 5 1 jars connected to the FIA system and allows on-line measurement of ammonia, nitrate and phosphate while performing batch reactions in parallel. Each of the four examples described was designed to study a particular aspect pertaining to Enhanced Biological Phosphate Removal (EBPR): the effect of acetate addition on aerobic P-uptake; the recovery of aerobic P-uptake after a disturbance; the interaction between denitrification and P-release when acetate is added at various rates; and the effect of stored PHB levels on denitrification by phosphate accumulating organisms. Copyright (C) 1996 IAWQ.